A variety of consumer products available today rely upon the use of wireless communication. Examples include cordless phones, garage door openers, remote controls for appliances, and remote controlled toys. A common motivation that drives manufacturers of these and similar products is minimizing the cost associated with providing the required wireless communication capability. Thus, techniques to minimize the cost of radio equipment for transmitting and receiving radio frequency signals while maintaining reliable communication are continuously explored.
Microsoft Corporation of Redmond Wash. is developing a product which has motivated several advancements in the art of low cost radio equipment. Generally stated, this product consists of a device capable of receiving control data and speech data from a control system and responding by sending status and sensor data to the control system. More specifically, Microsoft is developing a remotely controlled toy which operates within an interactive learning and entertainment environment for children. The interactive learning and entertainment environment consists of an audio/video presentation and a toy that moves and talks as though it is participating in the presentation.
The remotely controlled toy operates in conjunction with a program running on a control system or a source of control data such as a computer videotape player or broadcast equipment. The control system transmits speech data to the toy which enables the toy to talk. In addition, the control system transmits control data to the toy which enables the toy to move its arms, legs, etc. Finally, the toy is equipped with sensors which can detect certain activity such as a child squeezing the toys hand. The control system can request the toy to transmit the current status of the sensors. Although the technology and capabilities of a toy such as this are quite complex and sophisticated, there is a need to provide these capabilities at low cost. Therefore, there is a need for remotely controlling the movement and voice synthesis capabilities of a controllable device such as a toy.
In the development of this product, Microsoft is addressing several technical limitations. The first technical limitation is due to the limited amount of bandwidth that the FCC has allocated for low-cost, consumer products. Because several products utilize this bandwidth, there is a large potential for cross-talk and interference between these devices. In an effort to minimize interference from other devices and to minimize interfering with other devices, it is desirable to limit the amount of frequency spectrum used by low-cost consumer products. Thus, there is a need for a radio transceiver that makes efficient utilization of the available bandwidth.
One technique that addresses the need for a low cost, bandwidth efficient radio transceiver is using amplitude modulation (AM) radio equipment and modulating a carrier directly with digital data. This technique is beneficial for at least two reasons. First, AM radio equipment is inexpensive and is easy to manufacture. Second, by directly modulating the carrier with the digital data to be transferred, the costs associated with converting the digital data into an analog signal at the transmitter end and then back into digital data at the receiver end can be avoided.
Digital data is generally characterized as a series of binary voltage pulses that represent logical 1's and 0's. The digital data may represent a variety of information such as ASCII text or digitized voice. The use of digital data has become the defacto standard for storing information in computers and electronic devices. In consumer products similar to those described above, it is often necessary to transfer digital information from one device to another. For instance, in response to pressing the volume button on a remote control for a television, the remote control may transmit digital information to a receiver within the television. The television then interprets the received information and adjusts the volume accordingly.
Directly modulating the carrier with digital data is referred to as encoding the digital data onto a carrier signal. The encoding process includes mapping the data bits into signal elements that can then be transmitted. Several techniques are available for encoding digital data and each of these techniques has advantages and disadvantages. The choice of which encoding technique to use primarily involves a tradeoff between conservation of bandwidth or the minimization of data errors.
Common encoding techniques include: non-return to zero (NRZ), return to zero (RZ), bi-phase, pulse delay modulation, and multilevel binary. NRZ encoding produces a signal that is a direct representation of the digital data (i.e., the NRZ signal is at one level for a logical 1 and then transitions to another level for a logical 0). Advantages of NRZ encoding of data include the simplicity of implementing the technique as well as the efficient use of bandwidth. Disadvantages of NRZ encoding include the difficulty in maintaining synchronization and the presence of a DC component in the transmitted signal. Both of these disadvantages result in increasing the cost and complexity of the receiver. The bi-phase encoding techniques utilize at least one transition for each data bit transmitted. The advantages of bi-phase encoding include the ability to easily maintain bit synchronization and the elimination of a DC component. By eliminating any DC components, isolation between a receiver and transmitter can be realized by the use of transformer coupling of the signals. The main disadvantage of bi-phase encoding is the inefficient utilization of bandwidth.
Pulse delay modulation, which uses varying pulse widths to represent unique patterns of one or more bits, is particularly suitable for low cost transceivers. The advantages of pulse delay modulation encoding techniques include the simplicity of implementing the technique, the efficient use of bandwidth, and the ease of maintaining synchronization. In addition, because the information is conveyed by using pulse widths, the polarity of the signal can be made irrelevant in certain implementations. This helps to reduce any DC component in the signal and to simplify the design of the transmitter and receiver. Thus, an inexpensive AM receiver coupled with a microprocessor can be used to detect and decode pulse width modulated signals by counting the clock cycles between edge transitions.
The most prominently known pulse delay modulation technique is the Miller encoding technique. In the Miller encoding technique, a logical 1 is represented by a single transition at the midpoint of a bit interval. A logical 0 is represented by no transition unless it is followed by another logical 0, in which case a signal transition occurs at the end of the bit interval. In certain instances, the Miller encoding technique will result in using less bandwidth than NRZ or bi-phase encoding techniques. In addition, the Miller encoding technique results in at least one transition per two bit times and never more than one transition per bit. This allows the Miller encoding technique to provide some self synchronization capability. The disadvantage of the Miller encoding technique is evident when encoding the worst-case bit pattern ("110110110 . . ."). In the worst-case scenario, the Miller encoding technique can result in generating a signal with a significant DC component and one which requires a wider bandwidth than NRZ. Thus, there is a need for a pulse delay modulation technique which has an improved bandwidth performance in both the best and worst-case scenarios.
A second technological limitation is due to the cost associated with providing by-directional communication between two devices. Generally, bi-directional communication is provided by utilizing two channels, one channel for each direction. For example, cordless phones communicate with a base station in this manner. The use of two channels in a transceiver either requires the use of multiple synthesizers or a single synthesizer that can be tuned to multiple frequencies. Both of these requirements result in increasing the cost and complexity of a transceiver. One technique that has been used to provide bi-directional communication in a bandwidth efficient transceiver is to multiplex bi-directional communication onto a single channel. This technique helps to keep the cost of the transceiver down while improving the bandwidth utilization. Concerning the improved bandwidth utilization, this technique allows a transmitter and receiver to utilize the same frequency spectrum in half-duplex operation. Because dedicated bandwidth is not required for both transmitting and receiving, half-duplex operation requires half of the bandwidth that full-duplex operation requires.
Multiplexing bi-directional communication onto a single channel has some limitations. For instance, if a device is dependent upon the continuous reception of data in real-time, then it may not be practical to multiplex bi-directional communication onto a single channel. For instance, the process of reversing the direction of the channel, transmitting data over the channel, and returning the direction of channel to its previous state interrupts the reception of data in real-time. Thus, for devices requiring the reception of real-time data, the cost reduction and bandwidth efficiency of a half-duplex transceiver may not be realized. Therefore, there is a need for multiplexing bidirectional communication onto a single channel which in turn allows a device that is dependent upon the reception of data in real-time to operate in a half-duplex mode.
A third technical limitation involves the complexity of providing a high quality speech capability to a remotely located, controllable device. Typically, speech can be transmitted by modulating a carrier with the analog speech signal or by digitizing the speech and sending a digitally encoded carrier signal. To provide reasonable speech quality, both of these techniques require significant bandwidth. For instance, modulating a carrier with the analog speech signal will require on the order of 6-8 KHz of bandwidth. On the other hand, a digitally encoded speech signal will require a data rate of 8 Kbits/second or higher. Not only does this technology require significant bandwidth, but the cost of radio equipment that can meet these requirements is not suitable for a low-cost device. Therefore, there is a need for a method to provide high quality speech capability at a lower data rate and that allows the use of inexpensive radio equipment.